1. Field of the Invention
The invention relates to a method for producing oxidic sintered nuclear fuel bodies by mixing a base powder of UO.sub.2 or a mixture of UO.sub.2 and PuO.sub.2, and powder containing rare-earth oxide, i.e. SE.sub.2 O.sub.3 and, in particular, Gd.sub.2 O.sub.3 as well as by subsequent compacting of the mixture of powders to form compacts and densifying these compacts by sintering in a gas atmosphere with reducing action at a temperature in the range from 1500.degree. C. to 1750.degree. C. . Rare-earth oxide in the German language is "Selten-Erdoxid" and is designated "SE.sub.2 O.sub.3 ".
2. Description of the Prior Art
Allowed U.S. Application Ser. No. 431,615, filed Sept. 30, 1982, now U.S. Pat. No. 4,512,939 dated Apr. 23, 1985, 30, 1982 and German Patent No. 31 44 684, disclose compacting a mixture of a UO.sub.2 base powder which has a specific surface in the range of 2 to 4.5 m.sup.2 /g and/or a mean crystallite diameter in the range from 80 nm to 250 nm and Gd.sub.2 O.sub.3 as an additive.
From this compacted powder, oxidic sintered nuclear fuel bodies are obtained which contain a rare-earth element (SE), e.g. gadolinium as a neutron-physically burnable neutron poison, and the sintered density of which amounts to more than 93% of the theoretically possible density. In a nuclear reactor in operation, these sintered nuclear fuel bodies therefore liberate relatively little gaseous or highly volatile nuclear fission products. Fuel rods which contain sintered nuclear fuel bodies obtained by the known methods, therefore probably will only develop a slight overpressure in the fuel rod cladding tubes. Also, no shrinkage and local overheating of these sintered nuclear fuel bodies occurs during operation in the nuclear reactor, so that fuel rod defects are avoided.
The UO.sub.2 base powder used in the known method has a relatively small specific surface and/or a relatively large mean crystallite diameter. Base powder with these properties may be ungranulated uranium dioxide powder obtained directly by the so-called ADU process corresponding to "Gmelin Handbuch der anorganischen Chemie, Uranium, Supplemental volume A3, pages 99 to 101, 1981", or by the so-called AUC process according to "Gmelin Handbuch der anorganischen Chemie, Uranium, Supplemental volume A3, pages 101 to 104, 1981", which was given the relatively small specific surface and/or the relatively large mean crystallite diameter only by a correspondingly increased dwelling time of the powder under pyrohydrolysis conditions in the AUC process.
UO.sub.2 base powder which is used for the known method is therefore manufactured from the start according to quite definite processes, to make it conform to the required specific surface and/or the required mean crystallite diameter. Different starting powder which is already present and does not have the required specific surface and/or the correct mean crystallite diameter, can lead to difficulties in the known method.